The invention relates to a color filter layer of a cholesterically ordered material, in which the axis of the molecular helix of the cholesterically ordered material extends transversely to the layer. The invention also relates to a liquid crystal color display device of the reflective type, provided with such a color filter layer.
Color filter layers, also referred to as cholesteric mirrors, are known per se. They comprise a relatively thin layer of a liquid crystalline material having a cholesteric (or chirally nematic) order. The liquid crystalline molecules of this material have such a structure that they order to a spiral or helix-like structure in a solution--spontaneously or under the influence of given doping agents. After such a solution has been provided between two parallel substrates, this helix-like structure is aligned in such a way that the axis of the molecular helix extends transversely to the layer thus obtained. A better alignment of the helix is obtained if an orientation layer is provided on the facing surfaces of the substrates.
When unpolarized light is incident on such a color filter layer, that part of the light which "fits" the (dextrorotatory or levorotatory) direction and pitch of the helix is reflected, whereas the rest of the incident light is transmitted. If desired, this transmitted light is absorbed on an absorption layer which is present behind the color filter layer. The (reflected) color of the layer is mainly determined by the pitch of the cholesterically ordered material. Such filter layers may be used as coating layers or as mirror layers, for example for decorative purposes.
The known color filter layers have a major drawback. It appears that the color (=reflection band) of the color filter is strongly dependent on the angle at which the filter is viewed. This phenomenon is known as color shift. This color shift is caused by the strong angle-dependent anisotropic properties of the cholesterically ordered material. Such a color shift is unacceptable in a large number of applications.